So as a matter of FACT, how UNreliable is TCAS on the (say) 20 mile scale?
I shouldn't imagine that it should be considered as untrustworthy as Faire d'income might lead us to believe, as the returns are transponder based from the identified traffic.
Perhaps on the 160nm and 320nm scale, the veracity is degraded due to time lag, but on closer in/nearer scales, one might assume that unless there is a radical heading change on the part of either party, the returns will ACCURATELY define the almost exact proximation in real time terms.
And it is upon THESE visual id's that TCAS will issue an alert.
It is therefore with more than a little difficulty, that I try to understand the abject criticisms of F.W's posting - except possibly to think that some ATC'ers might feel their positions threatened!?